Where we discuss everything to do with rockets!

Month: October 2016

Hopefully, over the past few posts, I have convinced you that using chemical rockets to get to space is a pretty horrible way of doing it. And, just as I am certain that I will continue to post about this technology, I am certain that we will continue to use them, since it is really the only way to actually climb out of this gravity well that we call home (at this time!). But, my friends, trust me when I tell you that there is something better. Actually, there are a number of technologies that are being worked on that may be better. All of them have some really major issues, but it is good that we are trying.

This is the first post in a series that is going to explore some alternate ways of getting around the solar system and off of this rock. And, because I like you guys, I am not going to start off with #10 and work up to #1. I am going to start off with the craziest possible way of getting us into space. (There are crazier ways of getting around the solar system, though!) Let’s get started.

Once upon a time, there lived a guy named Gerald Bull. Yes, he was sort of short and stocky. And Canadian. Here is a picture of him:

Gerald Bull

Gerald Bull came up with a fantastic idea. In 1961 he bought a 16-inch battleship gun from the US Navy for about $2000. That isn’t 16 inches long, that is 16 inches in diameter. He moved it to Barbados and started running tests with it. He put atmospheric sensing instruments into the noses of the shells, and then fired them into the atmosphere. These shells were about 150 kg and could go to altitudes of about 100,000 ft, or about 20 miles. In the air. By the way, these “rockets” were called Martlets. His program was called the High Altitude Research Program, or HARP.

Ok, let’s step back for a minute and think about this. Bull was firing a gigantic cannon straight up in the air with things that weighted about 300+ pounds. I launch weather balloons. These go up to the same height with packages that weigh 12 lbs. Bull was doing some crazy stuff! Interestingly, there is really not a great way of sampling this part of the atmosphere, since it is really hard for airplanes to fly this high. Satellites can’t orbit this low because the atmosphere is incredibly “thick” there. So, rockets are about the only good way to take in situ measurements in this area (well, above about 100,000 ft, or 30 km to about 200+ km) of the atmosphere. Bull fired about 1,000 of these Martlets into the atmosphere in just a year or so.

But it really doesn’t stop there.

In 1963, Bull created the Martlet-3, which reached over 100 km altitude. He could launch a “rocket” that could go up to space for about $5000. Considering that rockets can cost over a million dollars that do the same thing, this is super freaking cheap.

He then extended the length of the cannon to about 110 feet with the ultimate goal of launching things into orbit. (The reason that you extend the length of the cannon is because you can get the force of the expanding gas for longer, allowing the projectile to accelerate for longer.) His idea was to build a rocket that would be shot up to about 100 km, and then the rocket would fire and take the payload into orbit. This would be extremely cheap, since the majority of the mass to get something into orbit is used up just to get up to the right altitude. If you can get the “third-stage” of a rocket up to 100 km altitude with a big gun, then it is super cheap to get to orbit! His Martlets got up to 180 km altitude for a world record that is still in existence.

The HARP 16″ gun firing a Martlet-3

Unfortunately, Bull never reached this goal. There was a ton of red tape, with the US and Canadian government involved. Bull did not really believe in red tape and so he left the program. Bull continued to love big guns and started working for some shady people, developing highly accurate guns that could be used by one country against another. Ultimately, he worked for Iraq in helping them develop the Scud missiles that were supposed to be used against Israel. It turns out that Israel doesn’t really like this type of behavior, and Bull ended up with a few bullets in him in March of 1990.

The moral of the story (besides “don’t screw with Israel”) is that we could actually use a big gun to get us to outer space and ultimately into orbit. We don’t even need gunpowder to do this anymore – we can use the same technology that drives super-fast roller coasters and trains: linear induction motors. This technology has led to the development of rail guns by the Navy. You seriously have to watch this video. This is a massive increase in our technological capabilities. Basically, you accelerate the “bullet” up to speeds of about 4,000 miles per hour in the barrel of a gun. Serious horsepower.

If we can use this technology to knock things out of the sky, why aren’t we using it to put things into orbit? That is a fantastically good question!

There are two problems with this idea (beyond Israel killing you for trying):

If you got something up to orbital speeds as it left the gun, it would slow down extremely quickly because of atmospheric drag. Really, you want to have it launched upwards, and when it gets well above the atmosphere, have it accelerate up to orbital speeds using fuel. This is somewhat complicated.

The accelerations that take place with this are just unbelievably horrendous. A human would be a pancake if they were launched like this. So, humans will NEVER be launched into space using a bug gun. Maybe a very very very long runway, but never something super efficient like a gun. But, supplies and fuel and other things like instruments could be launched into orbit using this technique.

There are researchers who are working on techniques that could be used independently or in conjunction with a big gun, so that you wouldn’t have to actually take fuel to get to orbit either. While that is interesting, it is no where as cool as the Martlet. Seriously. Gerald Bull. What a guy.

One of the big problems with rockets is their size. They need to be truly humungous to get anything into orbit. Interestingly, the reason for this was explained back before modern day rockets were even invented. A Russian scientist named Konstantin Tsiolkovsky described why rockets need to be really big way back around the turn of the last century (like 1900).

The graphic below helps to understand what is going on. Let’s say you want to lift a blue cube into space. The blue cube has some mass to it. In order to accelerate the blue cube up to some speed it takes a total of two bricks of red fuel. Let’s put some pretend numbers to this to make it a bit easier to understand. Let’s say that you want to reach a speed of 4, and using two bricks of red fuel will give you a speed of 1. That is too slow. So, we need more fuel.

Now the problem is that we have the blue cube plus two red bricks of fuel, which is more massive than just a blue cube. So, we will need even more fuel to accelerate this. In order to accelerate the blue cube plus two red bricks by 1, we will need four red fuel bricks:

We can keep going on this. Now we have a blue cube and 6 red bricks. In order to accelerate all of them by a speed of 1, you need 8 red bricks of fuel.

After that, we will be going at a speed of 3. We are very close to 4! To accelerate all of those red bricks (we have 14 now!) plus the blue cube by another 1, it takes 16 red bricks of fuel! Yikes! This is growing out of control!

For a rocket, what would happen is that the 16 red bricks would burn to allow the rest of the fuel plus the blue cube to be accelerated by 1. Then the 8 red bricks would fire, accelerating the 6 red bricks plus the blue cube by 1, giving a total speed of 2. Then the four red bricks would burn and accelerate the two red bricks and blue cube by 1, giving a total speed of 3. Finally, the last two red bricks would burn, accelerating the blue cube by 1 and giving a total speed of 4.

The point of all of the above is that the amount of fuel that you need grows very quickly, since you have to have more fuel to lift the other fuel that lifts the other fuel which lifts the other fuel, etc. Tsiolkovsky realized this more than 110 years ago and came up with a formula that describes this phenomena (of course, he knew calculus, which helps to explain things a bit). There are two forms of his equation:

They are exactly the same equation (but probably don’t look like it because of the “e” and the “ln”), but just re-arranged to allow two different questions to be answered:

If we need the rocket to change speeds by a certain amount (V), and the empty rocket has a given mass (Mempty), how much mass does the rocket have to have at the start (Mfull)?

If we have a given amount of fuel and a rocket that has a given mass (Mempty and Mfull), how much change in velocity (V) can we get out of the rocket?

One detail that I left out, which was talked about in the last post about chemistry, is that there is a term in the equation that represents the exhaust velocity of the rocket (Ve). If we take the top equation above, there are simplistically two terms of the right hand side: the exhaust velocity and the ratio of the full mass of the rocket to the empty rocket. What this multiplication means is that if you want to reach a given speed (V), you can use less fuel (smaller ratio of masses) if you have a larger exhaust velocity (Ve). The amount of fuel still exponentially increases (this is sort of what the “ln” means), but if you use a fuel with a higher exhaust velocity, you can use significantly less of that fuel. So, you want to get a fuel that will really leave the rocket with as much speed as possible. Then you can use less of it!

You can also use these equations to prove that a rocket with stages is much more efficient that a single-staged rocket. I won’t do this here, but you can think of it conceptually given the diagrams above. Let’s pretend that the black boxes around the fuel and blue cube are different stages of the rocket and that they have mass, which is pretty much exactly how it works. For the biggest rocket (with the blue cube and the 30 red fuel bricks), the rocket will be quite heavy and it will really be hard to get the fuel and everything up into the air. When we burn the 16 red bricks, we then get to drop the gigantic storage tank and some motors and plumbing and all sorts of stuff. The rocket then has significantly less mass. The next 8 red cubes have a MUCH easier job to do in this case, and they can accelerate the rocket much faster. The same is true when the 8 red cubes are done burning and the rocket drops the second stage with the motors and plumbing and stuff for that.

Rockets typically have three or four stages, each with smaller motors (or less motors) and smaller fuel tanks, just as illustrated above. The most efficient rocket in the world would destroy itself as it burned, having an infinite number of stages. That is quite difficult to engineer, though.

Chemical rockets that use fuel like this are about the only thing that we have ever used to get something off the ground. But, there are other methods. Some of them are just scary, and could get you killed by the CIA. Let’s talk about that next time.